Conventionally, a terminal device for mobile communication has been implemented by adding a wireless access interface to a mobile terminal.
However, a wireless access network connected by a wireless access interface has different coverage, bandwidth, communication charge, and the like, depending on its type. Accordingly, to continue communication wherever the terminal device moves, the terminal has been required that has plural interfaces to handle plural wireless access networks with different coverage. Equipping such plural wireless access interfaces on a terminal device undesirably increases its cost and size, which is particularly problematic when plural terminal devices are carried.
As a means to solve the problem, the following technique is disclosed in the literature “Research on Seamless Handover in Mobile Network (in Japanese)” (Taisuke Matsumoto et al., IEICE Technical Report, NS2002-323, March 2003). That is, a local area network (LAN) is formed with plural mobile terminals having a short distance wireless interface with lower cost and smaller size than a wireless access interface, and more than one router to which the mobile terminals connected through a short-distance wireless line, with a wireless access interface added thereto, where a mobile terminal other than the router connects to an external network through the router.
In mobile communication, a terminal device particularly needs to be used while switching routers loaded with wireless access interfaces with different coverage (e.g. cellular and wireless LAN) according to its location. A communication interruption occurring due to the switching during telephone conversation is problematic in communication quality, and a technique to avoid the problem is disclosed in the above literature.
In an IP (Internet Protocol) network, what is called a default router needs to be set when a terminal device in a local area network (LAN) communicates with a terminal device in an external network. Such a default router can be set automatically by the router to terminal devices through Neighbor Discovery for IP Version 6 (referred to as “ND” hereinafter), and the setting method is disclosed in the literature “Neighbor Discovery for IP Version 6 (IPv6)” (T. Narten et al., RFC 2461, IETF, December 1998). The method is described hereinafter.
In ND, a Router Advertisement message (referred to as “RA message” hereinafter) including the following information is transmitted from the router periodically or in response to a Router Solicitation message (referred to as “RS message” hereinafter).
(1) IP address of the router
(2) Period of validity of information included in the relevant message (Router Lifetime, referred to as “RL” hereinafter)
(3) Reachable Time (referred to as “RT” hereinafter)
A terminal device that has received this RA message adds the above information to its own default router list and neighbor cache. If plural routers are registered in the default router list, the terminal device selects the primary router from the entries in the default router list when transmitting to a destination not registered in the routing table. The primary router broadcasts an RA message with RL=0 when the router itself shuts down. Receiving the RA message with RL=0, the terminal device, if a corresponding entry exists in the default router list, deletes the relevant entry. According to the literature “Neighbor Discovery for IP Version 6 (IPv6)”, however, the default router transmits an RA message with RL=0 only when the router itself shuts down. This disables the terminal device to communicate because regular RA messages remain transmitted even when connection of the router to an external network is overloaded, or difficult or impossible due to decreased radio signal strength when connected through a wireless access interface. In order for the terminal device to switch the default router to another, the terminal device needs to recognize that communication with an external terminal device has been disabled, which requires a long time. Accordingly, when communication of the router with an external network is disabled because of a reason other than a shutdown, the communication is undesirably interrupted over a long time.
In a LAN having a conventional duplexed router, a method for switching from the master router to the backup router when a failure occurs in the master router is disclosed in Japanese Patent Unexamined Publication No. H11-261561. That is, the backup router requests a health check to the master router, and then the backup router is automatically switched to the master router unless a health check reply is returned from the master router. However, to use the method disclosed in Publication No. H11-261561, plural routers need to share the same MAC address, which is unusable in some wireless networks where authentication at the MAC layer is performed.
Particularly, for a local network connecting to an external network through wireless communication, the backup router needs to remain unconnected to save power consumption and to establish connection with the external network when switching from the backup router to the master. For this reason, the following technique has been proposed. That is, in order for the backup router to be switched to the master rapidly, the backup router starts establishing connection with the external network when the connection state of the master router becomes unstable.
In the conventional method described in WO 2004/093397, for example, the master router broadcasts to all the nodes in the LAN, an RA message with RL=0 as a routing stop message when executing a routing function is expected to be disabled due to overloaded connection of the router to an external network or due to decreased radio signal strength when connected through a wireless interface. Then, the backup router broadcasts to all the nodes in the LAN, a routing possible message if the own routing function is executable or to be executable in a given time allowing for processing time to connect to the external network. The routing possible message at this moment uses an RA message with a time period required to become executable set thereto, to notify of the executable time. Then, a node on the LAN that has received the routing stop message and routing possible message updates its own routing table and switches the default router for connecting to the external network after the notified executable time elapses.
However, in the method disclosed in WO 2004/093397, a node on the LAN waits for updating the routing table until the executable time described in the routing possible message elapses although the switching-destination router has actually completed connection with the external network, thus disabling rapid router switching. Meanwhile, the node updates the routing table when the executable time elapses although connection with the external network has not completed actually, thus failing to receive packets.
Further, in a LAN employing a connection oriented protocol such as Bluetooth (registered trademark), a node on the LAN cannot receive a routing possible message transmitted by the switching-destination router if the node has not established Layer 2 connection for transmitting IP packets with the switching-destination router, thus disabling normal router switching.
Even if a node during communication searches for a default router autonomously to attempt switching after receiving a routing stop message from the switching-origin router, link connection with the switching-destination router has not been established, and thus the routing table cannot be updated. Accordingly, even if connection of the switching-destination router to an external network is completed, the node cannot execute path switching until link connection with the switching-destination router is established.